Negaim, Chapter 10, Mishnah 7. The subject of this chapter is nesek, the tzara'as that appears on a person's scalp or within the area covered by his beard. Unlike other negaim, a nesek is not marked by any change in the color of the skin; it is defined by baldness. The Rambam describes it in Hilchos Tumas Tzara'as: the hairs growing in those places come away together with their roots, and the spot they occupied is left bare, and such a bare spot is never reckoned as a nesek unless it measures at least a square gris. Two things render a nesek tamei: a golden-colored hair (se'ar tzahov), and pisayon, which here means that the hairless area grows larger as hair on its border falls away as well. The mishnah before us examines a situation of two distinct nisokim positioned beside each other.
"Shnei nisokim zeh betzad zeh
veshitah shel se'ar mafsekes beineihem,
nifrats mimakom echad tamei,
mishnei mekomos tahor."
Two nisokim lie side by side, and a row (shitah) of hair runs between them and separates them. If that dividing row is breached in one place, so that additional hair is lost there, he is tamei. If it is breached in two places, he is tahor.
Breached in one place
The Rav lists the various points at which this ruling operates. When the breach is bemakom echad, whether it occurs upon the completion of the initial seven days of hesger, or during the period after that ptur, or upon the completion of the second seven days, or during the period after the ptur that follows them, the verdict is tamei, sheharei pasa hanesek: the nesek has grown. Losing further hair in this way is exactly what pisayon means in a nesek, and pisayon is the siman that produces hachlata.
Breached in two places
Picture instead the same dividing row giving way at both of its extremities, above and below. The result is a strip of hair left standing in the interior of the nesek, ringed by bare skin on every side. Hair in that position is called mevutzar, hair fenced in by the nesek around it, and such fenced-in hair functions as a siman of taharah. So when the row loses two hairs at each of its two ends and what survives is a line of hair in the middle, the ruling is mishnei mekomos tahor.
The size of the breach
The mishnah then asks, "kamah tehei hapirtzah?" - how large must the breach be? Mekom shtei se'aros, the space of two hairs on each side. That is the amount of hair whose loss counts as a breach of the dividing row.
Nifrats mimakom echad kegris
The mishnah concludes: "nifrats mimakom echad kegris, tamei" - if the row was breached in one place to the extent of a gris, he is tamei. Some understand that the mishnah mentions the measure of a gris here in anticipation of the next mishnah, which will present a case where a gris leaves the person tahor.
The Tiferes Yisroel supplies a sevara explaining why this final clause is not superfluous. After all, we have already been taught that losing two hairs from one side of the row is pisayon and produces hachlata, so what does the case of a full gris in a single place add? His answer: one might have reasoned that a gris-sized breach should fare no worse than a breach coming from both sides. Where two hairs are lost at either end, the strip left in the middle is mevutzar and brings taharah, because we regard the entire region as one nega. Admittedly, in our case the surviving hair is not surrounded and cannot serve to be metaher. Nevertheless, we might have supposed that when one nesek pushes into its neighbor to the extent of an entire gris, the two merge and the whole area is reclassified as a single fresh nesek requiring hesger, meaning the man would be shut up once more instead of being declared muchlat, and the growth would never be counted as pisayon. The mishnah rules otherwise: this is pisayon, and he becomes muchlat.
The mishnah thus draws a clear line through the case of two neighboring nisokim: as long as the hair between them is lost from a single side, the nesek has spread and he is tamei, however large that loss may be; but once the loss comes from two sides and leaves hair enclosed in the middle, that enclosed hair itself declares him tahor.